PRINCIPLES OF TOXICOLOGY - Biology East Borneo

296 CHEMICAL CARCINOGENESISevaluated for cocarcinogenic responses rather than attributing all of the activity to the chemical beingtested.Issues Associated with the Histopathological ExaminationIn some instances, perhaps more so in years past, the histopathological examination of the slides takenfrom the control animals have not been examined as rigorously as those slides taken from the animalsadministered the test compound. While at first it might seem that more attention should be paid to thoseslides where the potential change is anticipated, this can lead to results that are an artifact of theexamination. For example, if all animals during the test became infected by a viral organism, and ifthis infection affected the background cancer incidence in a particular organ of the animal, then placinga greater emphasis on the “exposed” slides might lead one to reach erroneous conclusions. In thissituation the pathologist might identify more tumors in exposed animals simply because of the moreextensive microscopic search of the exposed tissues even though equivalent numbers of infectioninducedtumors might exist in both control and exposed animals.Other aspects of the histopathological examination may affect the outcome of the study. Forexample, what organs should be examined? Should we evaluate organs like the Zymbal glands of ratsif humans have no anatomic correlate? What relevance should be attached to results where only benigntumors, or tumors that behave benignly, are elicited? What relevance should be attached to a chemicalthat increases the tumor incidence in one organ while decreasing the tumor incidences in other organs,particularly if the total cancer/tumor risk of the animal group does not increase? Should we attach thesame significance to these results? (Note: Here the extrapolation to humans would essentially be nonet changes in the population’s risk of cancer.) As only one chemical example of this phenomenon,PCBs, a chemical of considerable regulatory restriction and interest, has been observed in severalstudies to produce liver tumors in rats, and relatively low exposure guidelines have been developed forthis chemical on the basis of such data. However, two general findings in these studies were that thetotal tumor incidence in exposed animals was not increased (because the prevalence of other tumortypes were decreased) and that these tumors did not behave like malignant masses; in fact, the exposedanimals lived on average longer that did the control animals.One final facet of this issue is the fact that over the years the pathological descriptions (criteria forclassifying pathological changes as tumors) have evolved. This means that chemicals using moremodern descriptions might be viewed as having lower tumor incidences than they would if theirevaluation occurred in years past. While this difference does not affect whether the test was consideredto have produced a positive finding for carcinogenicity, it does affect the tumor incidence reported inthe test, which, in turn, affects the perceived potency of the chemical as measured by the cancer slopefactors derived from the tumor incidence that was reported. Thus, the perceived potency of a chemicalcarcinogen, as measured by its cancer slope factor, may differ according to which pathological criteriawere used.Dietary and Caloric RestrictionsOver the years we have come to realize that nutritional status and caloric intake during the test canaffect the test results. In most test protocols the rats are fed ad libitum; that is, they are given constantaccess to food in the cage. Given the already restricted activities that can occur within these cages andthe propensity of animals to eat as often as allowed, the animals tested under these conditions aregenerally obese animals during much of their lifetime. Studies with a number of different chemicalshave shown that obesity can inflate the final tumor incidence that is observed; that is, there are a numberof chemicals that, when administered at the same dose, will result in those animals placed on a normalcaloric or restricted caloric intake to have significantly lower tumor rates than observed in animals fedad libitum. Since ad libitum feeding is the general rule in chronic animal test procedures, the resultsof many studies have been inflated or possibly made statistically significant by the mere fact that theanimals were allowed to ingest more food than their bodies need. Similarly, some chemicals might